Transistor and method of making same



nited States Patent rites aan. 19, 195s, ser. No. $60,120 s claims. (ci.stv-ass) The present invention relates to semiconductor amplifier andoscillator devices generally referred to as transisn tors and to amethod of making the same, and, in particular, transistors useful athigh frequencies.

Extensive analytical and experimental studies of transistors disclosethree essential features which determine the upper frequency limit of atransistor, namely: the base width and the associated transit-timeeffects of the minority charge carriers, the base resistance and thecollector capacity. Early efforts to decrease all three parameterssimultaneously to achieve a high frequency transistor were concentratedon point contact types of transistors with extremely small spacingbetween the emitter and collector elements. The production of suchdevices, however, is very difficult and their mortality is extremely.high.

In junction transistors a reduction of all of these three parameters isnot considered possible because their various effects contradict eachother so that if one of these quantities is made small, another becomesvery large. As a result, a compromise is usually made with thetransistor having a short diffusion path on the one hand, and a smallarea of the collector junction on the other hand.

According to the prior art practices, there are several methods ofobtaining such a compromise. Usually an indentation is created in thesemiconductor material having a very thin wall at the bottom. It hasbeen suggested that such indentation be formed by a mechanical process,for example drilling, but in such instance the techniques are consideredcrude and cannot be monitored sufficiently accurate. Other techniquesfor producing such indentations have also been suggested, as for exampleetching, by means of an electrochemical jet stream, ultrasonic attritionor high pressure abrasion similar to sand blasting. The disadvantages ofall of these methods is that the diffusion path is established prior tothe creation of the junction, and, therefore, control during the ensuingmanufacturing process must be accomplished with high exactitude.

According to the present invention, a relatively small didusion path isestablished at the time the collector element is joined to thesemiconductor body, and subsequentiy the emitter element in the form ofa fine wire electrode is bonded at an optimum position with respect tothe collector element. The emitter wire preferably exhibits at leastpartial solid solubility in the semicond uctcr material and acts as acarrier for group Ill or group V impurities. ln addition, mechanicalstability is imparted to the finished transistor by means of anelectrical bonding process which forces the nodal point of the emitterwire to the semiconductor surface.

A general object of the present invention is to provide improved meansand techniques for making transistors in a relatively simple manner withthe resulting product having performance characteristics which arestable, uniform and reliable under those conditions met in practice.

A specic object of the present invention is to provide transistors whichmay be operated at higher frequencies than has heretofore been possible.

Another specific object of the present invention is to provide improvedmeans and techniques whereby a junction type of transistor is producedwith an extremely short diffusion path or base width.

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Another specific object of the present invention is to provide improvedtechniques whereby the adjustment of the elective diffusion zone isaccomplished in novel manner using an electrical formation process forachieving a transistor that may be operated at relatively highfrequencies.

Another specific object of the present invention is to provide improvedmeans and techniques whereby minority charge carriers injected by theemitter of the transistor upon the collector are focused in a novelmanner.

Another specic object of the present invention is to provide an improvedtransistor operable at high frequencies in which the junction has asmall area and provides a relatively small capacity.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. This inventionitself, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may be best understood byreference to the following description taken in connection with theaccompanying drawings in which:

FIGURE 1 is generally a transverse sectional view through a triode formof transistor embodying features of the present invention.

FIGURE 2 is a View in elevation taken generally in the directionindicated by the arrows 2 2 in FIGURE 1.

FIGURE 3 shows a modified form of transistor and corresponds to a viewof the same as in FIGURE 2, but in this instance a plated conductingsurface surrounds the collector, said surface contacting the base.

FIGURE 3A shows another modified form of transistor and corresponds to aview of the same as in FIG- URE 1, but in this instance a platedconducting surface surrounds the emitter, said surface contacting thebase.

FIGURE 4 is a sectional view through another modified transistor of thetriode type embodying the present invention and in this instance aplated conducting surface surrounds both the emitter and collector andcontacts the base.

FIGURE 5 is an elevational view of the bonded barrier transistor, asshown in FIGURE 4, taken generally as indicated by the `line 5-5 inFIGURE 4.

FIGURE 6 illustrates one step in the process of making transistorshaving the structures shown herein.

`FIGURE 7 illustrates in schematic form the monitoring or lmeasuringcircuitry and the means for bonding the emitter to the semiconductorbody.

lFIGURE 8 illustrates in schematic form an amplifier incorporatinganother modified transistor of the tetrode type.

Referring to the form of the transistor illustrated in FIGURES l and 2,such transistor is originally in the form of a slab 10 of semiconductormaterial as, for example, germanium or silicon. The collector element 12of, for example, indium or antimony is diffused or dissolved into oneface of the semiconductor body 1t) in accordance with important featuresof the present invention. A collector wire v13 is bonded to thecollector pellet 12 to provide a suitable terminal. The base element 14comprises a thin-layer of conductor material bonded to one edge of thesemiconductor body 10.

The emitter 15 is in the form of a tine wire which is bonded to theopposite fact of the semiconductor slab .-10 in an optimum position withrespect to the collector pellet 12. The nodal point of the bondedbarrier transistor is indicated at 17 and the frontal zone of the alloybarrier is indicated by the reference numeral 18.

In the manufacture of the transistors illustrated herein, the baseelectrode 14 and the collectorI pellet 12 are initially attached to thesemiconductor body 10, and the emitter wire `15 is bonded to the body 10in an optimum position after the collector pellet 12 is diffused ordisandinia solvedslinto the body 10.. The-base electrode 14 may beintheform of a metal platingand, as indicated above, may be connectedVto the body lil either before the collector pellet 12 is diiusedor-dissolved into the body lil, at.' the Sametime as, thecollectorpellet l2..is diihisedjor dissolved into the body iii, or aftersuch collector,V pellet 12.15. (unused er dissolved im@ the. body is.

In accordance with an important-feature ohtheptesent invention, arelatively small charge carrier diffusionpath in the order of .0001 isestablished at the time4 the4 collector' pellet l2 is` dilused ordissolved. into. theslab lil.r The collector pellet l2 originally maybein the, form of a small cylinderor sphere of indium, antirnony, orother appropriate group HI or Vl elementand is;

simply placed on one face of the slab` 10, vasindicatedin FIGURE 6,although it is understoodthat the particular shape of pellet, eithercylindrical or spherical, isnot considered critical, The assembly showninf FIGURE. 6. is placed in an oven Ztl withthe slab. resting on,lforsexample, .a shelf 20A in such ovenflii and. withfjthel pellet,1.2.f resting on the slab and the assembly,issubjectedto al temperatureat which. solid state diffusion. on. solution takes place at arelatively high rate, for example avtSOl() degrees ccntigrade forgermaniumwithout anyv pressure between the slab liland the pellet l2,otherthanthat pressure due to gravity forces acting onthepellet. The asnsembly in the oven is heatedV by radiation, conduction, convection,induction or any combinationoffsucltineans. At. the time of suchheating,v a, pontion, ofthe'pellet diffuses or dissolves into the slabto. the` extent shown in FIGURE i.

After the collector pel-let is-lthus diiusedV or: dissolved.

into the body, theassembly is allowed. to retuinto. no1:- mal ambienttemperatures and a basen-electrode 1,4.- may be then plated on one edgeot the. slab. Withthe collector 12 and base elementsle thus bondedto theslab. 19in-.f the semiconductor material,.the emitter. electrode;

15, in the form of a thin wire, is bonded. at .an optimum. position onthat face of the-slab,whichisoppositeto the-face into which thecollector Vpellet is difiusedor disn solved. The emitter wireV i5isfbondeyd to` such. falce; using resistance heating or anyother-l:ie,ati1: tg method.:v

The transistor characteristic which is monitoredis the.

current gain between the collector 13 and the emitter electrode i5, Le.the characteristic usually referred' to as alphaCE; and when suchcurrent gain is .highestgthe alternating current heating circuitincluding lthe Vsource Z6v is closed by the timed switch 25 to bond thewirevto the slab its at such optimum position. `When switch 2Sisfclosed, heating current for bonding purposes flows through the AC.source 26, through.switch25,` through the emitter wire i5, throughtheslab ltlandrthrough-the base element 14, and back to source 2,6.However, before the switch 25 is closed'to achieve thelbondingbe tweenelements i5 and lil, the emitter wire `iSisiirst placed in an optimumposition, vas mentioned previously, using conventional measuringequipment which includes a conventional monitor circuit .22 vand aconventional oscilloscope 24 for measuring or determining inyconventional manner such characteristic alphaCE. For this purpose, theother three input terminals ofi the conventional monitor circuit 22 areconnected respectively to vthe emitter wire 15 which is in contact ywiththe slab, the

base element le and the collectorrwire 13. The output of the monitorcircuitis appliedto,` for example, theV-ver-'f tical deection plates ofthe oscilloscope which incorporates conventional means for developingtime base cathode beam sweeps. When the emitter wire 15 is so placedthat the observed vertical deiiection on the cathode ray tube 23 is atmaximum, the timed switch 25 is closed to effect the bonding betweenwire i5 and slab 10.. l

By these expedieuts, the collector to base capacitance is relativelysmall and yet there are. relatively high current gains whichotherwisewould'be lowered' when, according to prior art practices, thearea of the collector junction is increased'due to the diii'culty ofproducing extremely small emitterjunctions by the. usual alloy process.

It is understood that usingthese techniques, a junction transistor maybe produced of either NPN 0r PNP type, depending upon the materials.us'ed. The collector may beof acceptor or donor-.material and.originally maybe a cylinder having a diameter 017.010. and aheight of.015". The base 10 may bein the form` of. a wafer or slab of either N orP'typefwitha thickness of 10913. The' dif! fusion of the collector intothe base may be accomplished by adequately controlled. thermal energyin` the form of radiation, convection, conduction, or induction appliedin suflicient amount andi'ora suflicient period of time until thefrontal zone of acceptor or donor semiconductor alloy penetration, asthe case may be, approaches the opposite side of the semiconductor waferto within approximately .0001" of the surface. A base region thicknessof this dimension. isv thus establishedy prior to the formation of theemitter.

Since the value of current gain at elevated frequencies is generallyproportionalwithinli'mits to the ratio of'collectorto emitter.. junctionarea the absolute size of the collector junction. area,fit is desiredthat the emitter junction area. be as s'inall as possible. Thus, theemitter i5'` is `a Wire drawn Vtofapproxirnately .903" diameter, and insuchv casethe emitter junction' area isa very small `fraction of theareaof the collector junction.

hetransistor shown in FIGURE 3 is identical with the'transistor ,shownin FIGURE 2, but, in this instance, the junction region is surrounded bythe electrode 26 which-.ismetal plated onto the semiconductor body 10,with such conductingplating being in contact with the base electrode 14,but not contacting the junction region itself.

In the modilied arrangement shown in FIGURE 3A, the emitter 15 issurrounded by the electrode 26A which is metal platedonto thesemiconductor` body titl, with such conductingplating being in contactwith the base electrode 14.

In Ythe arrangement shown in FGURES 4 and 5, the conductive .coatingsurrounding the junction region and plated on the semiconductor body 10extends downwardly sufficiently to contact the base electrode le at eachside of the slab 16.

ln FGURE 8 the conductive coating 36A surroundimg the junctionregionandplated onthe semiconductor body i0 does not extend to; thebaseelectrode but` is separated therefrom by the space 31. A wire 33soldered to the; conductive coating or plating Sti serves as a suitableterminal for connection in a circuit, for example thecircuit shown inFIGURE 8.

ln FIGURE tiA the transistor in the form ot' a tetrode functions as aradio frequency ampliiier and the base electrode 14 is grounded.Theemitter electrode l5 is connected 'to one terminaly of a highfrequency source 35, the other terminal of source 35 being connected torthe hot" terminal of the DC. sourceo and. having the other terminalgrounded. The collector electrode Viisv connected to one terminal offthe.primary winding 33 of output transformer 391, the other terminal vofsuch winding .38 being connected to the D.C. source Lttl which has oneterminal grounded.

rEhe electrode 36A which serves as a focusing electrode is connected tothe lungrounded terminal of D.C. source 36. This electrode 30A serves tofocus the minority charge carriers injected at the emitter, thusreducing transit time effects and collector capacity to a minimumthereby increasing the power gain of the device at elevated frequencies.

While the particular emboments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore7 the aim in the appendedclaims is to cover all such changes and modifications as fall Within thetrue spirit and scope of this invention.

I claim:

1. VIn the art of producing a transistor having a semiconductor body anda pair of spaced electrodes, diffusing one of said elements into saidbody, adjusting the position of the other element on the surface of thebody, observing an electrical characteristic of said transistor whilethe position of said other element is being adjusted, and bonding saidother element to said body when said characteristic is optimum.

2. In the art of producing transistors having a semiconductor body and apair of spaced elements, dilusing or dissolving one of said elementsinto said body, adjusting the position of the other element on thesurface of said body, measuring an electrical characteristic of saidtransistor while the position of said other element is being adjusted,and producing an electrical current flow between said other element andsaid body for a controlled time interval when said characteristic isoptimum, of sufficient intensity to bond said other element to thesurface of said body.

3. A transistor in the form of `a slab of semiconductor material with apair of opposite faces and a side edge, a collector pellet dilfused ordissolved into one of said faces, a thin wire bonded to the other facewith a portion of the thin wire in solution in the semiconductormaterial, a base electrode bonded to said edge, and conductive materialsurrounding said pellet and bonded in conductive relationship to saidbase electrode.

4. A transistor in the form of a slab of semiconductor material with apair of opposite faces `and a side edge, a collector pellet diffused ordissolved into one of said faces, a thin wire bonded to the other facewith a portion of the thin Wire in solution in the semiconductormaterial, a base electrode bonded to said edge, and conductive materialsurrounding said thin wire and bonded in conductive relationship to saidbase electrode.

5. A transistor in the form of a slab of semi-conductor material with apair of opposite lfaces and a side edge, a collector pellet diffused ordissolved into one of said faces, a thin Wire bonded to the other facewith `a portion of the thin Wire in solution in the semiconductormaterial, a base electrode bonded to said edge, and conductive material,surrounding said thin Wire and said pellet and bonded in conductiverelationship to said base electrode.

6. A transistor in the form of a slab of semiconductor material with apair of opposite faces and a side edge, a collector pellet diffused ordissolved into one of said faces, a thin Wire bonded to the other facewith a portion of the thin Wire in solution in the semiconductormaterial, a base electrode bonded to said edge, and conductive materialsurrounding said pellet and said Wire and bonded to said body innon-conductive relationship to said base electrode to provide a focusingelectrode.

7. A transistor in the form of a slab of semi-conductor material with apair of opposite faces and a side edge, a collector pellet diffused ordissolved into one of said faces, a thin wire bonded to the other facewith a portion of the thin Wire in solution in the semiconductormaterial, a base electrode bonded to said edge, and conductive materialsurrounding said pellet and bonded to said body in non-conductiverelationship to said base electrode to provide a focusing electrode.

8. A transistor in the form of a slab of semi-conductor material with apair of opposite faces and a side edge, a collector pellet dlused ordissolved into one of said faces, a thin Wire bonded to the other facewith a portion of the thin Wire in solution in the semiconductormaterial, a base electrode bonded to said edge, and conductive materialsurrounding said Wire `and bonded to said body in nonconductiverelationship thereto to provide a focusing electrode.

References Cited in the file of this patent UNITED STATES PATENTS2,713,132 Mathews et al. July l2, 1955 2,725,315 Fuller Nov. 29, 19552,750,542 Armstrong et al. .Tune 12, 1956 2,789,257 Donkersgoed et alApril 16, 1957 2,792,539 Lehovec May 14, v1957 2,793,332 Alexander et ali- May 2l, 1957 2,802,159 Stump Aug. 6, 1957 2,821,493 Carman Ian. 28,1958 2,840,770 Jackson June 24, 1958 2,842,668 Rutz `Iuly 8, 19582,850,688 Silvey Sept. 2, 1958

